Communication devices such as wireless devices are also known as e.g. User Equipments (UEs), terminals, mobile terminals, wireless terminals and/or mobile stations. Wireless devices are enabled to communicate wirelessly in a cellular communications network or wireless communications network, sometimes also referred to as a cellular radio system or cellular network. The communication may be performed e.g. between two wireless devices, between a wireless device and a regular telephone and/or between a wireless device and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Wireless devices may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The wireless devices in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another wireless device§ or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or Base Transceiver Station (BTS), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the wireless devices within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the UE. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the UE to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for UEs. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
Multi-antenna transmission and reception is an efficient means to improve data rates and capacity in wireless communications systems. A multi-antenna transmission has two partly separate aims: improving Signal to Interference and Noise Ratio (SINR) and sharing SINR. For example, in a cellular communications system, not only noise but also interference from other cells disturbs transmissions. The inter-cell interference is typically rather strong at the cell edge and considerably weaker closer to the cell center while the opposite is true for the received energy of the intended transmission. Consequently, improving the SINR by means of beamforming is well-suited for cell-edge UEs. On the other hand, the cell-center UE with high SINR may benefit more from sharing the SINR by means of Multiple-Input Multiple-Output (MIMO) transmission with spatial multiplexing. The number of simultaneously transmitted data streams a MIMO channel can support is commonly referred to as a channel rank and the actually transmitted number of data streams is correspondingly termed transmission rank. Thus, rank-one transmissions increase the coverage in terms of cell edge data rates whereas spatial multiplexing, i.e. rank larger than one, improves peak rates. 3GPP LTE Release-8, 3GPP TS 36.211, v8.9.0, chapter 6.3.3, permits up to four data streams, i.e. layers, to be transmitted in downlink operation. Before data in the form of modulated symbols is transmitted it is distributed onto the transmit antenna array in a process called precoding. In order to maximize the data transfer efficiency, the transmission rank should, just like other transmission parameters such as the Modulation and Coding Scheme (MCS), the channel code rate, and the precoder weights, be adapted to the channel and interference situation. To enable this adaptation, estimation of the channel and interference characteristics is required and often this estimation is performed at the receiving node, e.g. in an UE, which then reports the outcome of the estimation to the transmitting node. It is however also possible to perform the estimation at the transmitting node.
In downlink transmission in 3GPP LTE Release-8 the estimation of the channel and interference situation is performed at a receiving node, i.e. in a UE. Then the receiving node reports recommended transmission parameters to the transmitting node, e.g. a base station. Please note that it is also possible for the receiving node to be a base station and the transmitting node to be a UE. The report comprises a recommended transmission rank, also known as Rank Indicator (RI), a preferred precoder for this transmission rank, as well as an estimation of the channel quality, e.g. a Channel Quality Indicator (CQI). The CQI is calculated conditioned on the fact that the recommended rank and precoder is employed in the transmission. The transmitting node typically uses the reported information to select transmission parameters, but it has the right not to follow the recommendation and use a different set of transmission parameters. However, it is difficult for the transmitting node to determine whether the used transmission rank is the most suitable one under current conditions. Choosing an unnecessary low or high transmission rank leads to non-optimal performance of the wireless communications system.